This invention is directed to a cathodic electrocoating composition and in particular to a cathodic electrocoating composition containing an anticrater agent which significantly reduces craters and improves edge coverage of an electrodeposited film of the composition.
The coating of electrically conductive substrates by an electrodeposition process, also called an electrocoating process, is a well known and important industrial process. Electrodeposition of primers to automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an auto body or an auto part, is immersed in a bath of a coating composition of an aqueous emulsion of film forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from polyepoxide resins which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion. Pigment paste, coalescent solvents, water, and other additives are combined with the principal emulsion to form the electrocoating bath.
The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics of the tank, the immersion time, and the like. The resulting coated article is removed from the bath after a set period of time and is rinsed with deionized water. The coating on the article is cured typically in an oven at sufficient temperature to produce a crosslinked finish on the article.
Continuing problems with cathodic electrocoating compositions have been the lack of smoothness in the cured finish, the presence of craters in the cured finish, and the lack of edge protection or edge coverage of the composition. There have been various additives proposed in the art to address one or more of these problems. U.S. Pat. No. 5,356,960, for example, discloses an anticrater additive. However, this additive has a tendency to migrate to the surface of the electrocoating after baking, resulting in poor adhesion of primers or other coatings used in the automotive and other industries. U.S. Pat. No. 5,723,519 discloses an anticrater agent which does not migrate during baking and thus overcomes the disadvantages noted above and additionally is said to improves edge coverage. However, there is still a need for electrocoat compositions that have improved appearance, a lack of craters, and improved coverage at the edges of the coated substrate.